Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches). A volatile memory element retains data only so long as the integrated circuit is powered. In the event of power loss, the data in the volatile memory element is lost. For example, static random-access memory (SRAM) chips contain SRAM cells, which are a type of volatile memory element. Volatile memory elements are also used in programmable logic device integrated circuits.
Volatile memory elements are subject to a phenomenon known as soft error upset. Soft error upset events are caused by cosmic rays and radioactive impurities embedded in integrated circuits and their packages. Cosmic rays and radioactive impurities generate high-energy atomic particles such as neutrons and alpha particles. The memory elements contain transistors and other components that are formed from a patterned silicon substrate. When an atomic particle strikes the silicon in the memory element, electron-hole pairs are generated. The electron-hole pairs create a conduction path that can cause a charged node in the memory element to discharge and the state of the memory element to flip. If, for example, a “1” was stored in the memory element, a soft error upset event could cause the “1” to change to a “0.”
Upset events in an integrated circuit corrupt the data stored in the memory elements and can have serious repercussions for system performance. In certain system applications such as remote installations of telecommunications equipment, it is extremely burdensome to repair faulty equipment. Unless integrated circuits demonstrate good immunity to soft error upset events, they will be unsuitable for these types of applications.